PCK is an enzyme which reversibly catalyzes the reaction forming oxaloacetic acid by carboxylation of phosphoenolpyruvate. ATP-dependent PCK (EC 4.1.1.49) and GTP-dependent PCK (EC 4.1.1.32) are known. Plant PCKs are dependent on ATP and play an important role in the process of photosynthesis by which starch is formed from carbon dioxide.
On the other hand, C.sub.4 plants include mainly the plants belonging to family Gramineae originated in tropical zone, and are well adapted for strong sun light, high temperature and to shortage of water. More particularly, the rate of photosynthesis of C.sub.4 plants is twice that of C.sub.3 plants, and the photosynthesis is not inhibited by oxygen in the air. The photosynthesis of C.sub.4 plants does not reach saturation even if they are irradiated with a light having an intensity by which the photosynthesis of C.sub.3 plants is saturated. Further, the optimum temperature for photosynthesis of C.sub.4 plants is higher than that of C.sub.3 plants.
As mentioned above, PCK of plants plays an important role in photosynthesis. Thus, if a C.sub.3 plant is transformed so that it produces the PCK of C.sub.4 plant, it is expected that various effects may be obtained, such as increase in the rate of photosynthesis, efficient utilization of the sun light, and promotion of photosynthesis at a high temperature. However, so far, PCK gene of plants, needless to say C.sub.4 plants, has not been entirely sequenced.